Implementation of IAEA-AAPM code of practice for the dosimetry

Implementation of IAEA-AAPM code of practice for the dosimetry

1. M. Saiful Huq, PhD, FAAPM, FInstP
Dept. of Radiation Oncology, University of
Pittsburgh Cancer Institute and UPMC Hillman
Cancer Center, Pittsburgh, Pennsylvania, USA

Implementation of the IAEA-AAPM Code
of Practice for the dosimetry of small
static fields used in external beam
radiotherapy

2. Collaborators
•Yongqian Zhang, Ph.D.
•Min-sig Huang, Ph.D.
•Troy Teo, Ph.D.
•Kevin Fallon, M.S.
•Cihat Ozhasoglu, M.S.
•Ron Lalonde, Ph.D.

3. Contributors to IAEA TRS-483
Missing: Ahmed Meghzifene and Stan Vatnitsky

4. •The Code of Practice addresses the reference and
relative dosimetry of small static fields used for
external beam photon radiotherapy of energies with
nominal accelerating potential up to 10 MV. It does
not address other radiotherapy modalities such as
electron, proton and orthovoltage beams
TRS-483 CoP

5. •It provides a CoP for machine specific reference (msr)
dosimetry in a clinical high energy photon beams. It is
based on the use of a ionization chamber that has been
calibrated in terms of absorbed dose to water N
D,w,Qo or
N
D,w,Qmsr in a standard’s laboratory’s reference beam of
quality Q
o or Q
msr.
•It also provides guidance for measurements of field
output factors and lateral beam profiles at the
measurement depth
TRS-483 CoP

6. •Radiation generators: 10 cm x 10 cm field can be set
•Follow TRS-398 CoP or AAPM TG-51 or equivalent protocol
•Radiation generators: 10 cm x 10 cm ref field cannot be
set
•Define machine specific reference (msr) field, f
msr
•Dimension of f
msr field
•Should be as close as possible to the conventional reference
field

Reference dosimetry: msr field

7. Machine type msr field
CyberKnife 6 cm diameter fixed collimator
TomoTherapy 5 cm x 10 cm field
GammaKnife 1.6 cm or 1.8 cm diameter collimator helmet, all sources
simultaneously out
BrainLab microMLC add on For example 9.8 cm x 9.8 cm or 9.6 cm x 10.4 cm
SRS cone add-ons The closest to a 10 cm x 10 cm equivalent square msr field
achievable
•f
msr should extend at least a distance r
LCPE beyond the
outer boundaries of the reference ionization chamber
FWHM ≥ 2r
LCPE + d
msr fields for common radiotherapy machines

8. •How were the field sizes for the msr dosimetry
arrived at?
•Ask: What is the size restriction on an ionization
chamber for msr dosimetry?

msr fields: selection of chambers

9. •CPE conditions exist when one of the edges of the field
extends at least a distance r
LCPE beyond the outer
boundaries of the ionization chamber. If the size of the
detector is d, the FWHM of the field has to fulfil the
condition:
FWHM ≥ 2 r
LCPE + d
r
LCPE (in cm) = 0.07797•%dd(10,10)
x – 4.112
r
LCPE (in cm) = 8.369•TPR
20,10(10) – 4.382
d
r
LCPE
CPE condition for field size

10. •Consider a 6 MV beam. It’s TPR
20,10(10) = 0.677
•r
LCPE = 8.369  0.677 – 4.382 = 12.8 mm
•PTW 30013 Farmer type chamber:
•cavity length l = 23mm
•cavity radius r = 3.1 mm
•wall thickness t
wall= 0.057 g/cm
2

•With ρ (PMMA) = 1.19 g/cm
3
, t
wall= 0.48mm
•In the longitudinal direction, the chamber outer size will be d
l = l + t
wall =
23.48 mm (say 23.5mm)
•In the radial direction d
r = 2( r + t
wall) = 7.2mm
•As d
l > d
r, the largest detector size is d
l
•Eq. for FWHM yields a FWHM = 2 x 12.8 + 23.5 = 49.1 mm


For a PTW 30013 chamber: FWHM ≥ 4.9 cm
Example

11. •Chambers must meet specifications for reference class
ionizations chambers. Table 3 in the CoP
• Refers to chamber settling time, polarity effect, leakage,
recombination correction, chamber stability, chamber material
•f
msr ≥ 6 cm x 6 cm
•Chambers listed in Table 4 meet this criteria
msr fields: selection of chambers

12. •Farmer type chambers:
•WFF beams: Farmer type chambers listed in Table 4 meets this
criteria
•FFF beams use a chamber with a length shorter than the
length of Farmer type chambers
•If you have to use a Farmer type chamber a correction for the
non-uniformity of the beam profile should be used. For 6 MV
beam this can be about 1.5%
msr fields: selection of chambers

13. •For field sizes smaller than 6 cm x 6 cm similar
analysis led to the chambers listed in Table 5
(including Gamma Knife)
•These are chambers with volumes smaller than 0.3
cc (chamber length 7 mm)
msr fields: selection of chambers

14. •For reference dosimetry in msr fields, you will need to
determine “equivalent square msr field” sizes. For non-
square field sizes, the corresponds to the field for which
the phantom scatter is the same.
•Tables 15-17 tells you how to do this. This is needed to
calculate TPR
20,10(10) or %dd(10,10)
x using Palman’s
equation.

Equivalent square msr field sizes

15. 15
Table 15 (Tables 16 & 17 are for FFF beams)

16. •Choice of an appropriate detector for small field
dosimetry measurements depends on the parameter to
be measured.
•Note: NO ideal detector exists for measurements in small
fields
•Use two or three different types of suitable detectors so that redundancy in results can provide more assurance that no significant errors in dosimetry are made
Relative dosimetry: Detectors

17. •Assume that detectors used for large field dosimetry will not
perform well in small fields
•Ion chambers: major issues are volume averaging and substantial
perturbations in the absence of LCPE, signal to background ratio for
small volume ionization chambers
•Below certain field sizes, volume averaging effects become
unacceptably large. Below these field sizes only liquid ion chamber
and solid state detectors are suitable for dosimetry, but even those
exhibit substantial perturbations for the smallest field sizes
Relative dosimetry: Detectors

18. •Output correction factors are given as a function of
the size of the square fields. For non-square fields,
one determines a “equivalent square small field” for
which output corrections are the same
Output correction factor

19. •Field output correction factors are given as a
function of Collimator setting for CyberKnife and
Gamma Knife (Tables 23 and 25) and as a function of
“equivalent square” for Tomotherapy, MLC and SRS
cones for 6 and 10 MV beams in Tables 24, 26 and 27
Output correction factor

20. Table 23: Output correction factors for
CyberKnife

21. Table 26: Output correction factors for 6MV in
WFF and FFF beams

22. •For relative dosimetry ensure placement of the
detector in the center of the radiation field
Practical considerations

23. •Correct and incorrect orientations of detectors
for measurements of beam profile
Practical considerations

24. •The absorbed dose to water at the reference depth z
ref in water
for the f
msr field in a beam of quality as Q
msr and in the absence
of the ionization chamber is given by :


• is the chamber reading corrected for influence
quantities

• is the absorbed dose to water calibration coefficient
of the chamber at beam quality Q
msr
msr
msr
msr
msr
msr
msr
f
QwD
f
Q
f
Qw NMD
,,,
⋅=
Chamber calibrated specifically for the msr field
Formalism: Preferred option

25. •The absorbed dose to water at the reference depth z
ref in water for the f
msr field
in a beam of quality as Q
msr and in the absence of the ionization chamber is given
by:


• is the chamber reading corrected for influence quantities

• is the absorbed dose to water calibration coefficient of the chamber at
beam quality Q
0 in the ref field f
ref = 10x10 cm
2

• is a correction factor that accounts for the differences between the
response of an ionization chamber in the field f
ref and beam
quality Q
o and the field f
msr and beam quality Q
msr
Chamber calibrated in a conventional reference field and generic
values of beam quality corrections factors available
Formalism: Option b

26. Table 12: vs %dd(10,10)
x and TPR
20,10(10)
for WFF beams

27. Table 13: vs% dd(10,10)
x and TPR
20,10(10)
for FFF beams

28. Table 14: for GammaKnife

29. Beam quality
TPR
20,10(S) %dd(10,S)
x

30. •Equations for beam quality in non-standard reference
fields
(Palmans 2012 Med Phys 39:5513 )
0.55
0.60
0.65
0.70
0.75
0.80
0.85
2 4 6 8 10 12
s / cm
TPR
20,10
(s)
(b)
4 MV
10 MV
8 MV
6 MV
5 MV
25 MV
21 MV
18 MV
15 MV
12 MV
Beam quality
C=(16.15 ± 0.12) x 10
-3

31. •Field output factor relative to reference field (ref stands here for
a conventional reference or msr field)




where is the so-called output correction factor, which
can be determined as a directly measured value, an
experimentally generic value or a Monte Carlo calculated
generic value

Field output factor

32. •Method 1: Field output factor relative to msr field is given by




•Method 2: Field output factor relative to msr field using
intermediate field or ‘daisy chaining’ method





where

Assumed
to be unity
Field output factor

33. Field output correction factor

34. 34
Field output correction factor

35. •Rectangular small fields with uneven in- plane and
cross- plane FWHM, the equivalent square field size
is given by

S
clin = √ A.B 0.7 < A/B < 1.4

•For circular small fields with FWHM radius r

S
clin = r√ π = 1.77r


Equivalent square field size

36. Application of TRS- 483
What did we measure?

37. •TPR
20,10(10) and %dd(10)
x in msr fields for a 6MV
beam based on measurements in different field sizes
in a TrueBeam STx linac
•Polarity effect in a GammaKnife Perfexion machine
•Reference and relative dosimetry (Field output
factors) in GammaKnife Perfexion, CyberKnife M6
with Incise MLC and TrueBeam STx machines
Measured …..

38. msr Field
Size
(S)(cm
2
)
Measured
TPR
20,10 (S)
Calculated
TPR
20,10 (10)*

3x3 0.631 0.669
4x4 0.634 0.666
6x6 0.645 0.666
8x8 0.656 0.667
10x10 0.667 0.667
* Measured using CC13 chamber. Calculated
using Palman’s equation
0.6
0.62
0.64
0.66
0.68
0.7
3x3 4x4 6x6 8x8 10x10
TPR
20,10
Field Size (cm
2
)
Calculated TPR20,10 (10)
Measured TPR20,10 (S)
TPR
20,10(10) for 6 MV beam

39. * Measured using CC13 chamber. Calculated
using Palman’s equation
%dd(10,10)
x for 6 MV beam
msr Field
Size (cm
2
)
Measured %dd
(S)
Calculated %dd
(10,10)
X *
3x3 60.48 65.79
4x4 61.71 66.15
6x6 63.80 66.65
8x8 65.34 66.76
10x10 66.54 66.54
60
62
64
66
68
70
3x3 4x4 6x6 8x8 10x10
% dd
Field Size (cm
2
)
Calculated %dd (10,10)X
Measured %dd (S)

40. •Palman’s equation accurately calculates TPR
20,10(10) or
%dd(10,10)
x from measured values of of TPR
20,10(s) and
%dd(10,s) for various field sizes

Conclusion on Beam quality index
measurements

41. Polarity effect

Co-60 beam
16 mm collimator
3 Exradin A16 chamber, 1 Exradin A14 chamber, 1
PTW 31016 chamber, 2 Capintec PR05 P chamber

42. V (volts)

Polarity factor

0.97
0.975
0.98
0.985
0.99
0.995
1
1.005
1.01
0 100 200 300 400 500 600 700 800
A16SN100075
A16SN040907
A16SN031113
PTW31016
A14
PR05P9546
PR05P7837
Polarity effect
V (volts)

43. Reference dosimetry

44. GammaKnife® - Icon
TM

45. Chamber Serial
number
Dose rate
TRS-483
Dose rate
TRS-398
Difference
(%)
Exradin A16 040907 3.235 3.177 1.8
Exradin A16 092725 3.230 3.171 1.8
Gamma Knife (Icon)

46. CyberKnife M6
TM
Incise
TM
MLC

47. Chambers Dw(z
max)/MU
TRS-483 (cGy/MU)
Dw(z
max)/MU
TRS-398 (cGy/MU)
Ratio
TRS-398TRS-483
Exradin A12

1.016 1.009 0.993
PTW 30013
Sl. No. 1551

1.014

1.010

0.996
PTW 30013
Sl. No. 0262

1.018

1.013

0.995
PTW 30013
Sl. No. 0343

1.017

1.012

0.995
PTW 30013
Sl. No. 0905

1.024

1.019

0.995
Avg ± sd 1.018 ± 0.4% 1.012 ± 0.4%
CyberKnife : 6X FFF

48. TrueBeam
TM
STx

49. Chamb
er-
Serial #
M
correc
ted
(nC)

N
D,w
10
9
(Gy/C)
%dd(10,
10) x
D
w/M
U at z
max
TPR
20,10
(10)
D
w/M
U at z
max

D
w
(%dd)
x/
D
w(TPR)
PTW
30013-
1551

14.46

5.397

66.38

0.991

1.007

0.668

0.993

1.009

0.998
PTW
30013-
0262

14.57

5.343

66.38

0.991

1.004

0.668

0.993

1.006

0.998
PTW
30013-
0905

14.54

5.37

66.38

0.991

1.008

0.668

0.993

1.010

0.998
EXR A12
-020581
15.75 4.915 66.38 0.995 1.002 0.668 0.995 1.003 0.999
TRS-483 : 6X

50. Chamb
er-
Serial #
M
correc
ted
(nC)

N
D,w
10
9
(Gy/C)
%dd(10,
10)x
D
w/M
U at z
max
TPR
20,10
(10)
D
w/M
U at z
max

D
w
(%dd)x/ D
w(TPR)
PTW
30013-
1551

13.77

5.397

63.89

0.995

1.007

0.632

0.995

1.007

1.000
PTW
30013-
0262

13.88

5.343

63.89

0.995

1.005

0.632

0.995

1.005

1.000

PTW
30013-
0905

13.85

5.37

63.89

0.995

1.008

0.632

0.995

1.008

1.000

EXR A12
-020581
14.98 4.915 63.89 0.998 1.001 0.632 0.998 1.001 1.000
TRS-483 : 6X FFF

51. Chamb
er-
Serial #
M
correc
ted
(-nC)

N
D,w
10
9
(Gy/C)
%dd(10,
10)x
D
w/M
U at z
max
TPR
20,10
(10)
D
w/M
U at z
max

D
w
(%dd)x/ D
w(TPR)
PTW
30013-
1551

15.97

5.397

73.15

0.979

1.004

0.740

0.980

1.004

0.999
PTW
30013-
0262

16.13

5.343

73.15

0.979

1.004

0.740


0.980

1.004

0.999

PTW
30013-
0905

16.05

5.37

73.15

0.979

1.004

0.740

0.980

1.004

0.999

EXR A12
-020581
17.33 4.915 73.15 0.985 0.998 0.740 0.986 0.999 0.999
TRS-483 : 10X

52. Chamb
er-
Serial #
M
correc
ted
(-nC)

N
D,w
10
9
(Gy/C)
%dd(10,
10)x
D
w/M
U at
z
max

TPR
20,10
(10)
D
w/M
U at z
max

D
w
(%dd)x/ D
w(TPR)
PTW
30013-
1551

15.33

5.397

71.39

0.985

1.005

0.707

0.987

1.007
0.998
PTW
30013-
0262

15.48

5.343

71.39

0.985

1.005

0.707


0.987

1.007
0.998

PTW
30013-
0905

15.38

5.37

71.39

0.985

1.004

0.707

0.987

1.005
0.998

EXR A12
-020581
16.60 4.915 71.39 0.991 0.997 0.707 0.992 0.998 0.999
TRS-483 : 10X FFF

53. Chamber-
Serial #
TPR
20,10(10) D
w/MU at z
max

D
w(TRS398)/
D
w(TRS483)
PTW
30013- 1551

0.668

0.992

0.993

1.008

1.009

0.999
PTW
30013- 0262

0.668

0.992

0.993

1.005

1.006

0.999
PTW
30013- 0905

0.668

0.992

0.993

1.008

1.010

0.999
EXR A12
-020581
0.668 0.995 0.995 1.003

1.003 1.000
TRS-483 vs TRS-398 : 6X

54. Chamber-
Serial #
TPR
20,10(10) D
w/MU at z
max

D
w(TRS398)/
D
w(TRS483)
PTW
30013- 1551

0.632

0.996

0.995

1.008

1.007

1.001
PTW
30013- 0262

0.632

0.996

0.995

1.006

1.005

1.001
PTW
30013- 0905

0.632

0.996

0.995

1.009

1.008

1.001
EXR A12
-020581
0.632 0.998 0.998 1.001

1.001 1.000
TRS-483 vs TRS-398 : 6X FFF

55. Chamber-
Serial #
TPR
20,10(10) D
w/MU at z
max

D
w(TRS398)/
D
w(TRS483)
PTW
30013- 1551

0.740

0.980

0.980

1.004

1.004

1.000
PTW
30013- 0262

0.740

0.980

0.980

1.004

1.004

1.000
PTW
30013- 0905

0.740

0.980

0.980

1.004

1.004

1.000
EXR A12
-020581
0.740 0.986 0.986 0.999 0.999 1.000
TRS-483 vs TRS-398 : 10X

56. Chamber-
Serial #
TPR
20,10(10) D
w/MU at z
max

D
w(TRS398)/
D
w(TRS483)
PTW
30013- 1551

0.707

0.987

0.987

1.006

1.007

0.999
PTW
30013- 0262

0.707

0.987

0.987

1.006

1.007

0.999
PTW
30013- 0905

0.707

0.980

0.980

1.005

1.005

0.999
EXR A12
-020581
0.707 0.98 0.987 0.997 0.998 0.999
TRS-483 vs TRS-398 : 10X FFF

57. Relative dosimetry
Field output factor

58. CyberKnife

59. CyberKnife

60. TrueBeam STx

61. 61
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
6X
mean = 0.713
∆
uncorr = - 2%
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Uncorrected ratio of readings

Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
6X
mean = 0.728
TrueBeam STx : 6X

62. 62
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
6X (IMF)
mean = 0.715
∆
uncorr = - 2%
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Uncorrected ratio of readings

Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
6X
mean = 0.728
TrueBeam STx : 6X

63. 63
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
Uncorrected
Corrected
Corrected (IFM)
6X, PTW60017
0.5
0.55
0.6
0.65
0.7
0.75
0 0.5 1 1.5 2
Field output factors
Equivalent square field size, Sclin (cm)
Uncorrected
Corrected
Corrected (IF)
6X, PTW60017
mean = 0.591
∆
uncorr = - 3.6%
mean = 0.714
∆
uncorr = - 0.9%

TrueBeam STx : 6X

64. 64
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Uncorrected ratio of readings

Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
6X FFF
mean = 0.743
0.5
0.6
0.7
0.8
0.9
1.0
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
6X FFF
mean = 0.728
∆
uncorr = - 2%
TrueBeam STx : 6XFFF

65. 65
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Uncorrected ratio of readings

Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
6X FFF
mean = 0.743
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
6X FFF (IFM)
mean = 0.732
∆
uncorr = - 2%
TrueBeam STx : 6XFFF

66. 66
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
Uncorrected
Corrected
Corrected (IF)
6XFFF, PTW60017
0.5
0.55
0.6
0.65
0.7
0.75
0 0.5 1 1.5 2
Field output factors

Equivalent square field size, Sclin (cm)
Uncorrected
Corrected
Corrected (IF)
6XFFF, PTW60017
mean = 0.607
∆
uncorr = - 3.5%
mean = 0.730
∆
uncorr = - 0.6%

TrueBeam STx : 6XFFF

67. 67
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Uncorrected ratio of readings

Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
10X
mean = 0.692
0.5
0.6
0.7
0.8
0.9
1.0
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
10X
mean = 0.677
∆
uncorr = - 2%
TrueBeam STx : 10X

68. 68
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Uncorrected ratio of readings

Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
10X
mean = 0.692
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
10X (IFM)
mean = 0.678
∆
uncorr = - 2%
TrueBeam STx : 10X

69. 69
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
Uncorrected
Corrected
Corrected (IF)
10X, PTW60017
0.5
0.55
0.6
0.65
0.7
0.75
0 0.5 1 1.5 2
Field output factors
Equivalent square field size, Sclin (cm)
Uncorrected
Corrected
Corrected (IF)
10X, PTW60017
mean = 0.506
∆
uncorr = - 3.9%
mean = 0.675
∆
uncorr = - 1.5 %

TrueBeam STx : 10X

70. 70
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Uncorrected ratio of readings

Equivalent square field size, Sclin (cm)
PTW 60017
10XFFF
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
PTW 60017
10X FFF
TrueBeam STx : 10X FFF

71. 71
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Uncorrected ratio of readings

Equivalent square field size, Sclin (cm)
PTW 60017
10XFF
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
PTW 60017
10X FFF (IFM)
TrueBeam STx : 10X FFF

72. 72
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Field output factors
Equivalent square field size, Sclin (cm)
Uncorrected
Corrected
Corrected (IF)
10XFFF, PTW60017
0.5
0.55
0.6
0.65
0.7
0.75
0 0.5 1 1.5
Field output factors
Equivalent square field size, Sclin (cm)
Uncorrected
Corrected
Corrected (IF)
10XFFF, PTW60017
mean = 0.506
∆
uncorr = - 3.9%
mean = 0.675
∆
uncorr = - 1.5 %

TrueBeam STx : 10X FFF

73. •For the GammaKnife msr beam, differences in
references dosimetry using TRS-483 and TRS-398 can
be up to 2% assuming depth scaling is taken into
consideration
•For linac WFF and FFF beams, the values of Dw/MU
following TRS- 483 are consistent within better than 1 %
with those obtained using TRS-398
•The small field dosimetry of certain msr (reference) and
most relative (using field output factor) beams can be
significantly improved when the correction factors or
different detectors included in TRS-483 are appropriately
incorporated into their dosimetry
Summary